Field of the Invention
The present invention relates to a belt tension adjustment mechanism and an image forming apparatus using such a belt tension adjustment mechanism.
Description of the Related Art
A belt unit in which an endless belt is laid in a tensioned state by a plurality of rollers and which drives the endless belt in a circumferential direction is widely used as, for example, an intermediate transfer belt or the like in an electrophotographic image forming apparatus. In such a belt unit, when the belt is left in a tensioned state for a long period of time, creep (phenomenon in which when a sustained stress acts on an object, a distortion is increased with time) occurs, and this can cause an uneven image or the like. Hence, when the belt is not rotated, the belt is brought into a relaxed state by releasing the tension of the belt, and thus a creep deformation is prevented from occurring on the belt.
For example, Japanese Unexamined Patent Application Publication No. 2005-326011 proposes a tension adjustment mechanism in which a tension roller is provided on the side of the rotation end of a swing arm and in which when a belt is rotated, the swing arm is rotated in a direction in which a tension is applied to the belt by the tension roller whereas when the belt is stopped, the swing arm is rotated in a direction in which the tension of the belt applied by the tension roller is released.
Japanese Unexamined Patent Application Publication No. 63-188175 proposes a transfer belt support mechanism which includes a swing member and a force application member that applies a tension to a belt by the swinging of the swing member, and in which when the belt is rotated, the force application member applies the tension to the belt by the swinging of the swing member in a predetermined direction whereas when the belt is stopped, the tension of the belt produced by the force application member is released by the swinging of the swing member in the opposite direction.
However, in the technology proposed by Japanese Unexamined Patent Application Publication No. 2005-326011, since a torque limiter is used, a torque is increased by an amount corresponding to a torque resulting from the idling of a crutch. Moreover, disadvantageously, a space occupied by the tension arm is large, and thus the size of the apparatus is increased. On the other hand, in the technology proposed by Japanese Unexamined Patent Application Publication No. 63-188175, the mechanism is disadvantageously complicated so as to be expensive.
Moreover, although in an image forming apparatus, in order to prevent a color shift, it is necessary to keep integral multiples of the length of a belt, the diameter of a photosensitive member and the diameter of a drive roller, when the diameter of the drive roller is decreased so that the size of the apparatus is reduced, the creep deformation of the belt is increased. Hence, the creep deformation of the belt is a significant obstacle to the reduction of the size of the apparatus.
The present invention is made in view of the foregoing conventional problem, and an object of the present invention is to provide a belt tension adjustment mechanism in which a torque is low and the occupied space is small and which can adjust a belt tension with a simple mechanism and can prevent the creep deformation of a belt.